As most electrical power products, the development trend of a DC/DC converter is to form a converter with high efficiency, high power density, high reliability and low cost.
For a DC/DC converter outputting low voltage and high current, it is important to optimize the design of a transformer in order to meet the above-mentioned development trend. And due to the requirement of high efficiency at higher and higher output current, the secondary side coil of a transformer is changed from conventional winding type coil to a strip type continuous conductive planar type coil.
Please refer to FIG. 1, which is disclosed in U.S. Pat. No. 6,577,220 to show a perspective view of coil structure. As shown in FIG. 1, the coil structure 1 is made by folding a strip type continuous conductive structure. In comparison with the conventional winding type structure, the coil structure shown in FIG. 1 has a reduced DC resistance and an increased heat-dissipating area, so that the conduction loss of the transformer is significantly reduced.
For high power density, the switching frequency of a circuit is increased so as to reduce the volume of a magnetic component. However, there are some problems when the coil structure shown in FIG. 1 is used in a high frequency condition.
With the increasing frequency, the skin effect and the proximity effect of the AC current in a conductor are more severe, so that the AC loss is correspondingly increased. In addition, the electromagnetic radiation will generate from a circuit owing to the improper layout, so that the power density and the reliability are disadvantageously affected owing to the electromagnetic interferences.
When the coil structure shown in FIG. 1 is used in the high frequency condition, high frequency current with reverse directions flow through the output terminals (such as output terminals 11, 12) of the coil structure. When the output terminals are very close, current concentrates on the two sides proximate each other owing to the coupling of electromagnetic fields therebetween, which is the proximity effect, and therefore the current is distributed unevenly which increases the power loss. Furthermore, there is an interval 10 between the two output terminals 11 and 12, so that there is the electromagnetic interference produced from the interval 10 to surroundings, and the circuit is also interfered with the electromagnetic radiation in the surroundings.
Please refer to FIG. 2, which is a block diagram showing a DC/DC converter according to the prior art. In FIG. 2, the DC/DC converter includes an input circuit 21, a transformer 22 and an output circuit 23. In addition, there are two output terminals 24 and 25 on the secondary side of the transformer 22, and a loop 26 is formed by the output terminals 24, 25 and the output circuit 23. The current doubler rectifier circuit, the voltage doubler rectifier circuit, the full-bridge rectifier circuit and the half-wave rectifier circuit have the above-mentioned structures, and the characteristics thereof are those the current flowing into the output terminal 24 of the transformer 22 has the identical quantities and reverse directions with the current flowing out of the output 25 of the transformer 22, and the current in the loop 26 is high efficiency AC current. According to the electromagnetic field theory, the loop passing the high frequency AC current therethrough produces the high frequency magnetic field (magnetic field H in FIG. 2), which emits the electromagnetic wave to interfere surroundings. In addition, the electromagnetic radiation in the surroundings can also be received by the loop 26, so as to interfere the circuit itself. Therefore, in order to reduce the electromagnetic radiation in the surroundings and to reduce the interference in the circuit, the area of the loop 26 should be significantly reduced.
Please refer to FIG. 3, which is a block diagram showing another DC/DC converter according to the prior art. In FIG. 3, the DC/DC converter 3 includes an input circuit 31, a transformer 32 and an output circuit 33. In FIG. 3, the transformer 32 has a center-tapped structure, and there are three output terminals 34, 35 and 36 on the secondary side of the transformer 32. The first loop 37 is formed by the output terminals 35, 36 and the output circuit 33 on the secondary side of the transformer 32. The second loop 38 is formed by the output terminals 34, 36 and the output circuit 33 on the secondary side of the transformer 32. The third loop 39 is formed by the output terminals 34, 35 and the output circuit 33.
When the current is flowing in one (such as the loop 37) of the loops, the current flowing in the output terminal 35 and the current flowing out of the output terminal 36 have identical quantities and reverse directions. When the current flows in the loop 38, the current in the output terminal 34 and the current in the output terminal 36 have identical quantities and reverse directions. The current in the output terminal 34 and the current in the output terminal 35 have identical quantities and the phase difference of 180°. And the odd-order harmonics of the AC component of the current in the output terminal 34 and 35 have identical quantities and reverse directions. The center tapped full-wave rectifier circuit has the above-mentioned structure. Similarly, in order to reduce the electromagnetic radiation in the surroundings and to reduce the interference in the circuit, the areas of the loops 37, 38 and 39 should be significantly reduced, i.e. the outputs 34, 35 and 36 should be very close which can also alleviate the unevenly distribution of the current due to proximity effect.
Please refer to FIG. 4(a) showing a voltage type full-wave rectifier circuit according to the prior art. As shown in FIG. 4(a), the full-wave rectifier circuit 4 includes switches S1, S2 connected to the secondary side of the transformer T and connected to the output inductor L, wherein the output inductor L is connected to the positive terminal of the output capacitor Co, and the negative terminal of the output capacitor Co is connected to the center tap on the secondary side of the transformer T.
When the full-wave rectifier circuit shown in FIG. 4(a) is used in a pulse width modulation (PWM) circuit, the waveforms of the current passing through the switches and the center tap (i.e. the output inductor L) is shown in FIG. 4(b).
In FIG. 4(b), the current passing through the switch S1 is denoted as i1, the current passing through the switch S2 is denoted as i2, and the current passing the center tap is denoted as i3.
FIG. 4(c) is a diagram showing the harmonic spectra of the current i3. FIG. 4(d) is a diagram showing the harmonic spectra of the currents i1 and i2 passing through the switches S1 and S2. FIG. 4(c) and FIG. 4(d) show a ratio of the harmonic frequency to the switch frequency versus a ratio of the harmonic amplitude to the output current. As shown in FIGS. 4(c) and 4(d), the AC component of the current i3 is a very small part of the total current and has the even-order harmonics, and the odd-order harmonic of the current passing through the switches S1 and S2 is the major part of the total current.
Therefore, there is a need to provide different coil structures for secondary side rectification circuits of various transformers in order to overcome the aforesaid drawbacks of the transformer used in the high frequency condition.
Accordingly, in order to overcome the disadvantages of the prior art described above, the present invention provides a continuous conductive planar coil structure for a high frequency transformer.